Why Airplanes Don’t Always Fly in Straight Lines to Their Destination

If you’ve ever been on a flight equipped with a screen that shows the flight path, you might notice some zigs and zags that make your direct flight look like a scenic air tour. There are a number of reasons for this, but most of the time it comes down to Air Traffic Control (ATC).

Some people think that air traffic controllers are the guys that stand on the ground, waving lighted wands to guide the plane up to the gate. Those guys are actually part of the ground crew and they only have control over your flight for the last couple hundred feet before you reach the gate. The rest of the flight is controlled by someone else and it’s not the pilot.

The pilot flies the plane, but his course is being set by somebody on the ground. Those people are known as Air Traffic Controllers.

This system is a lot more complicated than it seems.

At the airport, the air traffic controllers sit up in the control tower. Those guys decide who gets to take off and land, which runways they use and when. They also direct planes that are moving around on the ground between gates and runways on the apron and taxiways. This aims to provide an organized flow of ground traffic and a safe flow of air traffic.

Once your plane has left the immediate area of the airport, the pilot must then communicate with a regional controller at an Area Control Center (ACC). If you’re on a long flight, you may get passed from one ACC to the next multiple times as you fly across the country.

Why?

The main purpose is safety. These controllers use radar to see all of the flights in their area and they are expected to provide proper separation between planes. Depending on the altitude, the minimum separation distance changes. That is because less sophisticated planes have pressure sensitive altimeters which are less accurate at higher altitudes — greater separation at higher altitudes compensates for risk.

At high altitudes (above 29,000 feet / 8870 meters), planes are required to have at least 2000 ft of vertical separation (it is usually 1000 ft when below 29,000 ft) although there are exceptions.

They even stagger the planes based on the direction they’re flying. Planes on the next level above and below you will becoming towards you. This makes it easier for the planes to see each other and ensure they’re at the proper height to avoid a collision.

Horizontal or lateral separation also varies. It’s not dependent on altitude but distance from the radar antenna. 3 nautical miles of separation are required within 40 nautical miles of the antenna, 5 nautical miles when more than 40 nautical miles from the radar antenna. This compensates for inaccuracy as the planes get further away from the radar antenna.

Many aircraft (especially large commercial planes) also have an on board system that is designed to alert the pilot when other planes are dangerously close or on a collision course.

When airplanes are over land, they actually have to fly where there is ground based radar that can track them. So your flight may be directed into “lanes” that keep the flights in an organized flow and within radar range. As I mentioned, having planes in radar view is important for preventing collisions but it’s also important for dispatching help if a plane stops responding or goes missing — they have an accurate last point of record.

Over the ocean, there isn’t any radar tracking. Many planes have on board GPS navigation systems, but there is no current standard to make that data available to flight control centers. This the why it was so difficult to find the Air France flight 447 that crashed in the ocean en route from Rio de Janeiro, Brazil to Paris, France in June 2009. Nobody noticed the flight was missing until it failed to make a radio call as it entered airspace controlled by a ground radar facility. When they tried to find the plane, the last location information they had was in a few short messages automatically relayed by the maintenance system on the plane as some systems began to fail before the crash.

The FAA (United States Federal Aviation Administration) is currently working on Nextgen, a new GPS based flight tracking system that will be much better than the current ground based radar network.

For now, some flights will let you listen in on the radio communication channels that pilots use to talk to controllers. If you ever have the chance to listen in, it’s very interesting. In fact, you can stream live feeds from LiveATC.net.

As a Geography teacher, I must also point out that the arc pattern that most planes fly is related to the fact that the earth is fatter closer to the equator, so flying in an arc closer to the poles is actually a shorter flight path than flying over the fatter part of the earth. This is why you fly awfully close to Greenland on the way from the East coast of North America to Western Europe or the UK. Even though it would appear as though the shortest route would be a straight line, it’s actually not. The shortest route is an arc.

Thanks Harper, that is a good point. I didn’t really talk much about the spherical shape of the Earth and what that looks like on flat maps. You’re right, the fastest route is directly between the two points, which is an arc on a flat map.

That said, it should be noted that planes can’t always fly in that perfect arc, sometimes they have to deviate to stay within high altitude flight paths that keep them away from other planes and within view of ground radar.

no sorry shortest routes still a straight line. as for the arc well that’s just due to actual altitude off of the ground jeeze and again tyhe quickest way between 2 points is a straight line not some crazy arc. and about planes flying close to greenland it’s because of the jetstream it helps them make more money off of you

I am a commercial pilot (and actually gainfully employed as such). I could quibble over your definition of Pilot in Command (ie: who is actually controlling the flight), or the alphabet soup acronyms used to define airspace and air traffic control, but I won’t. You have described global air traffic routing in a way that is understandable to the non-flying public, and for that I commend you!

My only complaint is with your explanation of “separation” based on the accuracy of altimeters.

>>>>because less sophisticated planes have pressure sensitive altimeters which are less accurate at higher altitudes — greater separation at higher altitudes compensates for risk.<<<<WWII) have pressure-sensitive altimeters. ALL aircraft of U.S.Civil Registry, and ALL aircraft of ICAO (International) Registry have the same quality of altimeters. The display might vary — digital or analog — but the accuracy is routinely tested and required to meet identical standards, regardless of the “sophistication” of the aircraft. These tests are required every 24 calendar months and are entered in the aircraft logbooks.

It is true that greater vertical separation is usually required at higher altitudes. However, this is NOT due to possible altimeter inaccuracies. It is simply due to the higher SPEED of aircraft capable of flying at those altitudes. Just like you give yourself more space driving on the Interstate than in a parking lot, separation requirements are based on speed.

In the older systems it is true that separation above 29000ft was 2000 ft. This was most definitely because of inaccuracy of altimeters at high altitudes. As you are very aware, the rate of pressure drop as we climb higher falls. At 15000 feet itself, pressure drops at about 1hpa/45feet vs the 1hpa/30feet at sea level. For this reason, the system was originally developed to allow for this accuracy.

In the new system called RVSM or Reduced Vertical Separation Minima, this separation is reduced to 1000ft even above 29000ft. This is because there are many new technologies which allow greater accuracy in altimeters and height measurement. Be it even devices like the GPS or Radar Altimeter which could be used to give you height data. Since altitude is now being reported much more accurately, it is much safer to have a less separation between aircraft even at very high altitudes.

The reason for higher separation was not speed as you put it. You will probably fly at a same Mach number whether you are at 26000ft or at 35000ft. Agreed your ground-speed will be different at both these altitudes but I do not think it will differ by more than 50-75 knots.

While staying in Radar range is important, one of the main reasons that aircraft fly in such weird lines is that they are flying from one navigational facility to the next. In airplanes without GPS or an inertial reference system, aircraft must travel from and to these navigational sites on the ground in order to get to the destination, with a minimum of input from an aircraft controller.

Need to know that why two aircrafts with less sophisticated altimeters need to maintain 2000 feet vertical separation from each other above 29000 feet?
What happens to these altimeters at this particular altitude?